View or print this publication - Northern Research Station - USDA ...

However, hares showed strong preference for it. The general trend in our experiments has been that species and genotypes
resistant to hares showed better than average growth, although the relationship was not statistically significant (Figs. 3 and 4).
These results support our earlier findings which indicate either a positive relationship between first year growth of birch
seedlings and their hare resistance, or no relationship at all (Rousi et al. 1989, 1990, 1991, 1996b).
European white birch genotypes have not shown any relationship between their growth and resistance to hares,
Microtus voles, Curculionidae weevils, or Melampsoridium leaf rust (Poteri and Rousi 1996, Rousi et al. 1996b). These
experiments did not support any ecological trade-off either: resistance to any one pest was not related to resistance to others.
However, vole and hare resistance in some experiments have shown a positive relationship (Chiba and Nagata 1969), and
have also tended to do so in our own experiments (Rousi et al. t989, 1996b). Interestingly, clone 39 has shown generally
high resistance to both biotic and abiotic threats (Fig. 5, Rousi et al. 1996b). The same clone also seems to be exceptionally
resistant to ozone injury (clone 5-M, in P_ifikk6nen et al. 1993), whereas the generally herbivore susceptible clone 36 is
similarly very susceptible to ozone damage (2-M, in P_i_ikk6nenet al. 1993).
Environment May Sometimes Affect Birch Resistance
Our previous experiments with various birch species (8 birch species) and origins have indicated, as expected, that
growth environment (shading or fertilization) does not have an effect on low resistance species (slow or fast growing), such
as B. papyrifera, pubescens, maximowicziana, schmidtii or grossa. In high resistance species, fertilization increased the
palatability of two species, B. platyphylla and B. ermanii, but did not affect another, B. resinifera. Shading had no clear
effect on any of the tested species (Rousi et al. 1996a).
European and Japanese white birches are closely related (Dugle 1966). However, fertilization did not diminish the
hare resistance of European white birch (Rousi et al. 1991, 1993), but lowered that of Japanese white birch (Rousi et al.
1996a). On the other hand, shading tends to increase hare palatability of European white birch (Figs. 6 and 7), but its effect
on Japanese white birch is negligible (Rousi et al. 1996a). The effect of shading on European white birch is, however,
strongly dependent on genotype and site fertility. There were no statistical differences in resistance among three families that
we tested in low fertility soils (Fig. 6). But, if seedlings are grown in high fertility soil, resistance was significantly different
in two out of three families (ANOVA, Bonferroni t-test, p